Digital synthesis of microwaves uses transmission lines and switches to generate a series of alternating polarity pulses. The coupling of the resulting pulse train to a load such as an antenna results in the radiation of a short microwave pulse. This approach has been investigated for over 30 years.
The general concept of producing microwaves by a sequential operation of switches is well known. High peak power microwave generation is addressed by Driver et al. in U.S. Pat. No. 4,176,295 in which the generation of microwaves by periodically discharging a plurality of identical, direct current energized, resonant transmission lines into a TE wave guide at multiple half-wavelength spacing is discussed. To periodically discharge the transmission lines, each line is provided with a switch, and all switches are simultaneously operated to cause the electromagnetic energy in the waveguide to propagate as a pulse train of microwave energy.
Mourou, in U.S. Pat. No. 4,329,686 discusses an arrangement, similar to that of Driver et al., which uses a TE waveguide and a light activated solid state (LASS) switch for generating microwave pulses of picoseconds duration, synchronously and in response to laser light pulses.
Unfortunately, the arrangements described by Driver et al. and Mourou do not produce clean microwave pulses and are limited in power since TE waveguides have impedances close to that of free space, typically 50 ohms or more, and therefore cause the LASS switches to operate outside the electric field and current density limits consistent with good high power design principles, specifically, unidirectional power flow in a continuously matched system.
Zucker, in “Light Activated Semiconductor Switches,” UCRL Preprint, October 1977 discusses the use of a light-activated semiconductor switch, the basic principle of which is to create carriers in situ, thus obviating the need for diffusing the carriers necessary to transition a transistor or thyristor switch from a reversed biased (OFF) condition to a forward biased (ON) condition. Zucker discusses the use of a laser beam whose frequency is matched to the switching device band gap (1.09 eV for silicon).to turn ON a LASS switch in less than 1 ps. As discussed in the article, a switch having sub nanosecond turn on time, and capable of being turned off after current ceases to flow, would be required for microwave generation in order to allow for quick recharge and refire and for the establishment of coherence among independent microwave sources.
Such a switch is addressed by Proud et al. in their article “High Frequency Waveform Generation Using Optoelectronic Switching in Silicon” IEEE Trans on Microwave Theory and Techniques, Vol. MTT-26, No. 3 (1978), in which the conversion of dc energy into RF pulses by using an array of silicon switches simultaneously activated by a laser pulse is discussed. Proud et al. describe a “frozen wave” generator comprising arrays of high-resistivity silicon switches fired by a gas laser designed to simultaneously fire all of the switches in synchronism. Both Zucker and Proud techniques are represented by FIG. 1, which discloses a group of transmission lines connected together by switches. In Zucker, the switches are activated sequentially, which gives flexibility in resulting wave shape, while in Proud the switches are activated simultaneously and produce frozen wave pulses. In both, the switches remain in the ON state during the transmission of the entire pulse train through the closed switches.
Mourou et al. in their article entitled “Picosecond Microwave Pulse Generation”, Appl. Phys. Letters. 38(6) (1981) discuss the generation of a microwave burst in picosecond's synchronization with an optical pulse using a LASS switch coupled to an x-band waveguide and describe the efforts of others to generate microwave pulses using electrically driven spark gaps and frozen wave pulses.
In U.S. Pat. Nos. 5,109,203 and 5,185,586, Zucker et al. teach:
(1) Sequential switching of two or more cascaded TEM transmission lines of arbitrary lengths, each transmission line being charged to an arbitrary voltage where the delay between any two switching events is equal or greater than the temporal length of the transmission line separating them with the first switch activated (closed) being the one closest to the load.
(2) The use of an optimized transmission line and switch geometry to yield the highest possible power flow.
(3) A “folded” microwave source configuration to provide added compactness and simplified energizing of the transmission lines.
(4) The use of reverse biased light activated solid state diodes as switches to provide for extremely rapid switch recovery upon recharging of the transmission lines after discharge, the recharging operating to forcefully reverse bias the diodes.
Despite the above advantages, the implementation of a transmission line as a series of segments coupled together by switches causes problems when trying to provide a number of pulses in series. This is because each pulse within sequential switching systems or frozen wave systems travels through several closed switches implemented in series. Therefore, the signal level attenuates as the signal propagates through each closed switch due to the residual resistance of each closed switch. Thus, sequential switching systems are not desirable for certain applications because of attenuation problems and are limited by a low number of pulses. Therefore, energy delivered to resistive load is relatively low.
A circuit called a Blumlein generator (U.K. Patent N/589127, 1941), depicted in FIG. 1B, has been used and based on a voltage inversion principle to generate power. The Blumlein generator operates using two identical two conductor lines. They can be incorporated in a single three conductor transmission line. In the latest version, the central conductor is charged to a voltage (V) relative to each of the outer two conductors. A single switch connects the central conductor to one of the outer conductors. When the switch is closed, the voltage on the switched line is inverted and, after a time equal to the delay of this line, both lines start to discharge to a load, converting the full stored potential energy into power on the load during double transit time of the line.
The Blumlein generators may be implemented in a stacked configuration (for increasing power) to enable the conversion of power from more than two transmission line segments. This is shown in FIG. 1C as one of the options for two stacked Blumlein generators. The stacked Blumlein generators, like the conventional Blumlein generator, generate a single unipolar pulse when all switches are closed (at the same time) that drives the load after the equal time delay of each line. Neither the Blumlein generator nor the stacked one, however, has been used for digital synthesis or to generate microwave signals in a series of bipolar pulses on the common load.
Related patent applications by Oved Zucker disclose digital synthesis of microwaves based on transmission line conductive segments where the transmission line conductive segments are coupled to continuous transmission-line conductors by closing switches which are connected in parallel with transmission line conductors, rather than in series. See co-pending patent application Ser. No. 11/147,453. This permits the number of transmission line segments or transmission-line sections to be increased. However, when the number of sections increases substantially, the full electrical (and physical) length of the multi-sectional generator also increases proportionally. With the increase in physical and electrical length, however, conductive losses associated with generating pulses traveling to the load also increase and the efficiency of the generator decreases. Therefore, even in configurations with switching elements positioned in parallel to transmission lines, conductive losses can quickly predominate and practically limit the number of sections implemented. As a result, there is limited power and energy delivered to the load.
To improve efficiency and increase energy at the load, the characteristic impedances of both transmission lines in each section should be increased. However, for a given fixed maximum voltage on each switch, this causes the power/energy to be decreased and there will be some optimum for the selected number of sections and particular specifications depending on the implementation. In all cases, to achieve the high power/energy on the load, the number of section is a principal limitation.
Accordingly, there is a need to increase the power, energy and efficiency of digital synthesis systems. There is a further need for a digital synthesis system capable of creating long pulses efficiently. There is still a further need for a digital synthesis system capable of maximizing the power and efficiency for a variety of multi-cycle pulse length and power and pulse requirements.